Hello TRX,
Check Nelson's Submachineguns of the World (Vol 1) for the Reising drawings. (my darn HP scanner doesn't work).
The Reising model 50 is a delay blowback .45 smg of WW-II fame. The bolt is cylindrical in a thick-wall steel tube receiver. The bolt tilts up at the tail end into a recess area in the receiver tube. The tilting is caused by a small cam edge on the op-rod working on the bottom side of the receiver through a slot.

The op-rod is also the cocking device and is spring loaded. It is not gas activated. It is a simple delay system. The op-rod is a welded-together fork-like steel strap thing. They sometimes broke at the weldment but can be rewelded better with today's processes. The Garand op-rod is also a welded assembly by the way.

The levers are actually the trigger sears that release the op-rod and striker hammer (closed bolt system).

The Reising is a simple and unique design that could have been an assault rifle had there not been such opposition to an AR round by US military brass.

My idea for a trunnion would be a simple block to hold the AR barrel extension. It wouldn't necessarily be any longer than the barrel extension, though it might be worthwhile to put a couple of "wings" on it to more-easily attach some rivets.

Press the AR extension in, then pin it in place in normal AK style. The AR barrel would then screw in.

The AR bolt is weird-looking. I may have to just buy one so I can inspect it. Meanwhile, it might be possible to turn/grind it down to AK size on the stem and press on a collar to engage the AK bolt carrier cam track. If not, doing a custom bolt carrier would be the next logical step. Then you'd be independent of the three major parts you still have to source from AK kits.

In the furtherance of taking on more projects than I can handle, I ordered a chunk of 1-3/4 x 1-3/4 1018 steel a while ago.

At the moment AR bolts cost about the same as AK bolts, and a brand-new barrel extension is $20-ish, vs. $50-ish for used AK bits.

The trunnion and extension setup looks like a no-brainer, but the bolt/carrier setup will require some thought.